Our continued studies on the mitogenicity of PPD-Tuberculin have shown that a subpopulation of B lymphocytes from uninfected, non-immune mice are stimulated to proliferative and produce antibody to unrelated antigens when exposed to tuberculoprotein in culture. When B lymphocytes are derived from mice sensitized to tuberculin by infection with Mycobacterium bovis (BCG), enhanced or depressed DNA synthesis is obtained depending on how BCG was given. Our main objective is to reveal the mechanism underlying these types of responses to PPD by investigating the interrelationships between B cells, T cells, and macrophages derived from BCG-infected mice. Also we intend to continue our studies of PPD stimulation of nonspecific immunoglobulin synthesis in vitro and in vivo, as well as the adjuvant effect of PPD on specific antibody synthesis.